One method for continuously coating thin layers of a liquid composition on a moving substrate such as a continuous web is the so-called curtain coating method. An early description of a curtain-coating method and apparatus for use in the manufacture of photographic film and paper is found in the patent to Hughes, U.S. Pat. No. 508,947, incorporated herein by reference. This and subsequent patents relating to curtain coating (e.g., U.S. Pat. No. 3,632,374 and U.S. Pat. No. 4,287,240) describe the use of a coating hopper to form a free-falling curtain of liquid photographic coating composition which impinges transversely across a moving web of film or paper and forms a coated layer thereon.
As the earlier patents indicate, curtain coating has capabilities which make it preferable to other coating methods for many products. Thus, although bead coating, as disclosed, for example, in the patent to Beguin, U.S. Pat. No. 2,681,294, the patent to Mercier et al., U.S. Pat. No. 2,761,419, the patent to Russell, U.S. Pat. No. 2,761,791 and others, is a valuable process, higher coating speeds are attainable in curtain coating. Also certain coating defects are more readily avoided in curtain coating, as explained in U.S. Pat. No. 3,632,374, cited above.
Despite its important advantages, curtain coating, presents the problem that the falling curtain can be disturbed by ambient air currents with resulting defects in the coated product. A solution to this problem is offered in the patent to O'Connor, U.S. Pat. No. 4,287,240. The patent describes forming a shield of a foraminous material such as screening or a perforated plate around the coating apparatus. The foraminous shield diffuses air currents to reduce their velocity and protect the curtain. Another solution is offered in the co-pending U.S. Pat. No. 5,114,759, issued to Finicum et al., filed Jul. 12, 1991, incorporated herein by reference.
Still another characteristic of curtain coating that can be a problem is that the curtain tends to bend back as it falls from the hopper or die. This is the so-called "teapot" or Coanda effect, which is discussed in the patent to Ridley, U.S. Pat. No. 4,135,477. Because of the bend-back characteristic, if it is desired to have the curtain impinge at top-dead-center of the coating roll over which the web passes, as in FIGS. 4 and 6 of the patent to Greiler, U.S. Pat. No. 3,632,374, the hopper lip must be positioned somewhat forward of that position. However, the correct position for one set of coating conditions, e.g., flow rates and viscosities, may not be suitable for other conditions. This can require movement of the hopper to move the line of impingement, which is often difficult. Furthermore, even when coating conditions are such that the same bend back will occur, it may be desirable to change the line of impingement from top-dead-center to another position without having to move the hopper.
A further problem in curtain coating is the tendency of the curtain to contract laterally under the influence of surface tension as it falls from the coating hopper. This is discussed, for example, in the patent to Reiter, U.S. Pat. No. 4,830,887, incorporated herein by reference, which describes the use of edge guides to prevent contraction of the width of the falling curtain. Edge guides fix the positions of the edges of the curtain. However, the teapot effect will control the position of the curtain between the edges. Typically, when vertical edge guides are used, the line of impingement of the curtain is curved rather than a straight line, with the center of the curtain impinging on the substrate at a point substantially to the rear of the edges. This can cause uneven thickness of the coating and other undesired results. A need exists, therefore, for a curtain coating method and apparatus in which the line of impingement of the curtain on the moving substrate can be maintained as a straight line or can easily be moved backward or forward as desired.